Do we excrete what we eat? Analysis of stable nitrogen isotope ratios of human urinary urea

Rationale: Natural stable nitrogen isotope ratios (δ¹⁵N) are frequently used for the determination of provenance and dietary assessment of recent and ancient humans. Although individual δ¹⁵N values typically correspond to the dietary δ¹⁵N composition, they are also affected by metabolic conditions. Preferred matrices for the measurement of human δ¹⁵N values have been hair, nail or blood. The goal of this study was to validate a novel approach for the assessment of the δ¹⁵N values from urinary urea, the principal end-product of human N metabolism. Methods: The method, which involves the precipitation of urea from urine using xanthydrol, was validated using fortified urea solutions. Intra- and inter-individual variance of the δ¹⁵N values of urinary urea was determined from samples obtained from multiple human subjects. Results: Precipitation with xanthydrol did not alter the δ¹⁵N values of urea. The mean δ¹⁵N value in urinary urea from human subjects from Germany was +4.4 ± 0.6 ‰, which corresponds to the estimated dietary composition. It falls below previously reported δ¹⁵N values for human tissue and blood samples. Longitudinal analyses over 7 days illustrate short-time changes linked to varying protein intake. Conclusions: Our results indicate that δ¹⁵N values can be measured reliably from human urine and that the method is suitable to monitor rapid dietary and metabolic changes of an individual. Our findings further confirm that urinary urea is depleted in ¹⁵N compared with human tissue but within the range of the δ¹⁵N composition of the diet.